High-frequency apparatus



June 3, 1952 N. R. wlLD 2,599,033

HIGH-FREQUENCY APPARATUS Filed Nov. 15, 1946 Oggi/522%@ Mae/vfmo/v 4 asc/Amro@ Patented June 3, 19512 HIGH-FREQUENCY APPARATUS Norman R. Wild, East Natick, Mass., assigner to Raytheon Manufacturing Company, Newton, Mass., a corporation of Delaware Application November 15, 1946, Serial No. 703,937

3 Claims. (Cl. 219==4) This invention relates to high-frequency apparatus, and more particularly to a steam and vapor shield for a microwave cooking device.

It has been found that, when food or other material positioned adjacent the open end of a hollow wave guide or horn is being heated by microwave energy propagating down the horn and out the open end thereof, several adverse effects are produced if the steam or vapor from the food is allowed to enter said horn.

In the first place, if microwave energy propagating down the horn impinges on clouds of steam therein, part of the energy is reected from the steam, thus not reaching the food or at least causing appreciable distortion of the heat pattern produced by the energy in the horn. This heat pattern should be kept uniform or free from distortion if the desired even heating of the food is to be achieved.

In addition, steam or other vapors cause corrosion of the horn, which corrosion is, of course, undesirable.

Moreover, if steam is allowed to enter the horn, contacting the inner walls thereof, there will be an appreciable loss of heat from the food, through conduction by the steam from the food to the walls of the horn. This heat loss is undesirable because it prevents efficient utilization of the available energy for heating the food.

Furthermore, if steam is allowed to enter the horn, it will condense therein, producing moisture in the horn, on the inside thereof. Moisture in the horn greatly reduces the energy transmission efficiency of the same, because such moisture both reflects energy and absorbs energy, thus reducing the energy output (at the food end of the horn) relative to the energy input to the horn.

It is, therefore, an object of this invention to mask the interior of a hollow wave guide from steam or other vapors while at the same time permitting microwave energy to pass through the mask.

Another object is to provide a means for preventing entry of steam into the horn of a microwave cooker, this means not interfering with the normal energy-transmitting function of the horn. A

2 cooker provided with a masking means in accordance with my invention; and

Fig. 3 is a section taken along line 3-3 of Fig. 2, on an enlarged scale.

Now referring to the drawing, and particularly to Fig. 1 thereof, a magnetron oscillator I is shown schematically as comprising a central cathode 2 surrounded by a conducting envelope 3 and a plurality of radially-extending vanes 4 which together constitute the anode structure. As is well known, such an oscillator is adapted to produce an output of high-frequency energy, for example in the microwave region of the frequency spectrum. The radio-frequency energy output of the magnetron is coupled to a coaxial transmission line 5 by means of a suitable loop 5 which extends into the magnetron i between a pair of adjacent vanes fi. The inner conductor 'l of the transmission line may, for example, extend into one end of a hollow wave guide or horn 8, in order to serve as an exciting rod for said guide, thereby transferring high-frequency energy from line 5 to the guide.

Wave guide 8 may be square in cross-section, as shown, or it may have any other desired crosssectional conguraticn. Guide 8 is open at its lower end, so that it may be termed either a wave guide or a horn. Radio-frequency energy supplied to horn 8 at its upper end is adapted to propagate down said horn, radiating into space from the lower open end thereof. A container 9, which has the same cross-section as horn 8 and which is open at its upper end, is positioned adjacent the open end of horn 8, a body of food l0 to be heated or cooked being placed inside said container in such a position as to receive radiofrequency energy from the horn.

Radio-frequency energy is propagated down horn 8 and radiated from the lower open end thereof, impinging on food body l@ and heating the same. As a natural consequence of this heating, clouds H of steam or other vapor are given off from the food and travel upwardly as shown, through the open end of the horn e and into said horn. As can be seen from an examination of Fig. l, there are clouds of steam in the horn 8, which produce the adverse eifects described in detail above.

Fig. 1 shows the resulting steam in the horn when no steam masking or shielding means is used. Now referring to Fig. 2, a device embodying a steam shielding or masking means according to my invention is shown. In this figure, parts the same as those of Fig. 1 are denoted by the same reference numerals. A solid continuous flat plate-like member I2, having an area slightly larger than the cross-sectional area of horn 8, is secured to said horn inmediately below the lower open end of said horn, covering the open end of said horn. Member I2 is made of a material which is substantially transparent to radio-frequency energy, such as quartz, for example, If desired, the area of member I2 may be the same as the interior dimensions of hom 8, and said member may have any desired thickness. Member I2 may be held in position over the mouth of the horn by any suitable means. For example, three elongated brackets I3 are secured to the horn 8, one to each of three sides thereof. The lower end of each of these brackets is shaped to embrace a corresponding edge surface of plate I2, holding the upper surface of said plate tightly against the lower surfaces of the walls of horn 8. To assemble plate member I2 on horn 8, said member is slid into place on brackets I3 from the side of the horn on which there is no bracket, the plate being held firmly in place due to the ytight engagement of the upper surface thereof with the lower surfaces oi the walls of the horn.

Container Ill holds food body I in position beiow member i2, the member I2 therefore .being positioned between seid body and the open end of guide or horn 8. In order to provide for the escape oi steam from food I0, container I4 may 'be provided with a foraminous side wall I5.

Member l2 is transparent to radio-frequency energy, as stated above, so that energy propagating down horn 8 can pass therethrough to heat body i0. However, said member is not transparent to steam or other vapors II, so that these cannot pass through plate I2 into the interior oi horn but instead pass harmlessly out Vinto the external atmosphere through foraminous wail l5, as indicated in Fig. 2.

The plate f2, since it masks or shields the in tcrior of the horn from steam, helps to keep the pattern at the food uniform or free from distortions which would be caused by steam in the horn. The plate also keeps the steam from corroding the horn, and keeps moisture from coni" ,ing in the horn. The plate prevents heat 4from being transferred, by the steam, from the food to the walls of the h-orn. 1n fact, all of the adverse eiiects which result from the presence of steam in the horn are entirely eliminated by the r esence of piatc I2, which is transparent to o-freduency energy but is opaque to steam, in a masking position at the mouth of the horn.

Oi course, it is to be understood that this invention is not limited to the particular details described above, as many equivalents will suggest themselves to those skilled in the art. For erample, any suitable fastening means other than that shown may be used to maintain plate I2 in position on horn Also, any desired material which is suitable may be used for plate I2. Various other variations wiil suggest themselves. It accordingly desired that the appended claims be given a broad interpretation commensurate with the scope of this invention within the art.

What is claimed is:

1. In a microwave cooking device, a hollow metallic wave guide having an open end, means for supplying microwave energy to said guide,

means for supporting an energy-absorbing body closely adjacent the open end of said guide to receive energy therefrom, said body producing vapors when the same is heated, and a vitreous masking member covering the open end of said guide, said member being substantially transparent to microwave energy and being positioned between said body and the open end of said guide, and lsaid supporting means being provided with openings for allowing said vapors to escape therefrom, said opening being less than one-eighth of a wave length of the frequency of said microwave energy.

2. In a microwave cooking device, a hollow metallic wave guide having an open end, means for supplying microwave energy to said guide, means for supporting an energy-absorbing body closely adjacent the open end of said guide to receive energy therefrom, said body producing vapors when the same is heated, and a vitreous masking member covering the open end of said guide, said member being substantially transparent to microwave energy and being positioned between said body and the open end of said guide, said supporting means being metallic and substantially surrounding the end of said guide and being provided with openings for allowing said vapors to escape therefrom, said openings being less than one-eighth of a wave length of the frequency of said microwave energy.

3. In a microwave cooking device, a hollow metallic wave guide having an open end, means for supplying microwave energy to said guide, means for supporting an energy-absorbing body closely adjacent the open end of said guide to receive energy therefrom, said body producing vapors when the same is heated, and a vitreous masking member covering the open end of said guide, said member being substantially transparent to microwave energy and being positioned between said body and the open end of said guide, and said supporting means being provided with openings for allowing said vapors to escape therefrom, said opening being less than one-eighth of a wave s length of the frequency of said microwave energy REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,317,883 Meacham Oct. 7, 1919 2,304,958 Rouy Dec. 15, 1942 2,355,670 Naeher et al Aug. 15, 1944 2,392,511 rThompson et al. Jan. 8, 1946 2,407,690 Southworth i Sept. 17, 1940 2,408,032 Beck Sept. 24, 1946 2,427,094 Evans Sept. 9, 1947 2,473,251 Hsu June 14, 1949 2,495,429 Spencer i Jan. 24, 1950 2,500,752 Hanson et al. Mar. 14, 1950 OTHER REFERENCES Hutcheson, Electronic Torch," The Welding Engineer, December 1945, page 90. 

